Duplication of genetic information and its partitioning to progeny cells are fundamental to all eukaryotes. Many lines of evidence suggest that oncogenes and tumor suppressor genes belong to the hierarchy of genes that regulate these processes. Oncogenes are normally positive regulators of the cell cycle and when activated, represent a gain of function in the cell. In contrast, tumor suppressor genes are negative regulators and promote transformation through their loss of function. While the number of oncogenes discovered continues to increase, the number of families to which they have been assigned has not. This may be due to the limited number of assays available for their detection, but it may also indicate that most of the families have been identified. The assignment of oncogenes to families was originally based upon their function, structural and sequence homology, or product localization, but the families appear to be taking on a new significance in the relationship with participation in the cell cycle.
Recent studies of signal transduction pathways in somatic cells have linked the products of one oncogene family either directly or indirectly to the activation of members of other families. For example, the stimulation of certain growth factor receptors by their appropriate growth factor or ligand results in the association of receptors directly with the src and raf products (Morrison et al., Cell, 58, 649-657 (1989); Kypta et al., Cell, 62, 481-492 (1990)). The receptors also associate with several proteins involved in second message pathways (e.g., PLC.gamma., PI3 kinase) (Coughlin et al., Science, 243, 1191-1194 (1989); Kumjian et al., Proc. Natl. Acad. Sci. USA, 86, 8232-8236 (1989); and Margolis et al., Cell, 57, 1101-1107 (1989)) as well as with a GTPase activating protein (GAP) that enhances the activity of the ras gene product. (Kaplan et al., Cell, 61, 125-133 (1990); Kazlauskas et al., Science, 247, 1578-1581 (1990)). Mitogenic stimulation of certain tyrosine kinase growth factor receptors results in specific transcriptional induction of a well-characterized series of genes, several of which are nuclear oncogenes. (Rollins et al., Adv. Cancer Res., 53, 1-32 (1989); Vogt et al., Adv. Cancer Res., 55, 1-35 (1990); Bravo R., Cell Growth & Differentiation, 1, 305-309 (1990)).
In contrast, however, understanding how such diverse gene families elicit expression of the transformed phenotype has not been so obvious. The fact that the members of these families function in the same or parallel pathways begins to address the problem of assigning hierarchy and determining whether a particular family is "upstream" or "downstream" in the pathway. It is obvious that growth factors or, for that matter, nuclear transcription regulators cannot be proximal effectors of the transformed phenotype. Assuming that most of the oncogene families have been identified, the most likely candidates for proximal effectors would be members of the kinase oncogene family, since they might modify nuclear and/or cytoskeletal proteins necessary for induction of morphological alterations associated with the neoplastic phenotype. Knowledge of such hierarchy is important for it may provide a means to develop strategies to intervene in neoplastic transformation.
Another major question is how these genes influence cell cycle. Restriction points in the cell cycle regulate entry into S-phase and M-phase and these control points are present in all species from yeast through man. The gene products that mediate and control these restriction points are being characterized. The cell cycle has been intensively studied in the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe. These yeasts are as distant from each other in evolution as they are from mammals. In spite of this, certain cell cycle regulators are conserved not only in structure, but also in function. Thus, CDC28/cdc2 genes from budding and fission yeasts are functionally equivalent. The product of this gene is a serine kinase whose targets are influenced during the cell cycle by the appearance of proteins termed cyclins. Cyclins, so named because of their cyclic appearance during M-phase of the cell cycle, were first discovered in clams and sea urchins. Independently, an activity termed maturation promoting factor (MPF) was discovered in unfertilized amphibian eggs (Masui et al., J. Exp. Zool., 177, 129-146 (1971); Smith et al., Dev. Biol., 25, 233-247 (1971)) as the activity responsible for inducing meiotic maturation (Masui et al., Int. Rev. Cytol., 57, 185-292 (1979)). MPF was subsequently found in all M-phase cells undergoing meiosis or mitosis from yeast to man and is therefore considered the universal regulator of M-phase in eukaryotes (Kishimoto et al., Exp. Cell Res., 137, 121-126 (1982); Kishimoto et al., J. Exp. Zool., 231, 293-295 (1984); Tachibana et al., J. Cell Sci., 88, 273-282 (1987)). MPF is responsible for nuclear envelope breakdown and chromosome condensation (Lohka et al., J. Cell Biol., 98, 1222-1230 (1984); Lohka et al., J. Cell Biol., 101, 518-523 (1985); Miake-Lye et al., Cell, 41, 165-175 (1985)). Lohka et al. (Proc. Natl. Acad. Sci. USA, 85, 3009-3013 (1988)) first purified MPF, which was subsequently shown to consist of the amphibian homologs of the yeast p34.sup.cdc2 gene product and cyclins (Gautier et al., Cell, 54, 433-439 (1988); Gautier et al., Cell, 60, 487-494 (1990)). Thus, in just a few years, an extraordinary series of discoveries allowed characterization of the major cell cycle regulator in species as diverse as yeast and man. The relationship between p34.sup.cdc2 kinase and oncogenes or tumor suppressor genes is emerging.
There remains a need for techniques to identify suitable anticancer drugs and treatments and for new and efficacious methods and pharmaceutical compositions for the treatment of cancer in mammals, particularly humans. It is an object of the present invention to provide such techniques for identifying suitable anticancer drugs and treatments. It is another object of the present invention to provide methods and pharmaceutical compositions for the treatment of cancer.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.